The present invention generally concerns the field of determining unbalance of a motor vehicle wheel and more particularly relates to an apparatus for driving a motor vehicle wheel while it is mounted on the motor vehicle, for the purposes of determining unbalance thereof.
Unbalance measuring machines for determining unbalance of a motor vehicle wheel while it is still mounted on the motor vehicle are generally in the form of easily manoeuvreable units, the use of which is particularly desirable and meaningful in situations such that removing the wheel from the motor vehicle would involve a relatively large amount of work. Such circumstances arise for example when measuring the unbalance of the heavy wheels of motor buses or trucks. Such an unbalance measuring machine can also be used to detect whether the wheel while still mounted on the motor vehicle has any substantial unbalance at all or whether, if the wheel suffers from a minor degree of unbalance, an improvement in the degree of smoothness with which the wheel rotates can be achieved by eliminating the minor unbalances to be found thereon. The unbalance measuring machine of that kind may also be appropriately used in a situation in which a wheel which has already been balanced on a stationary balancing machine has to be checked and possibly rebalanced again after it has been fixed to the motor vehicle in question, as it frequently happens that, when a motor vehicle wheel which has been balanced separately is then fitted to the motor vehicle, it may suffer from unbalance arising for example out of parts of the vehicle which rotate with the balanced wheel.
In order to carry out an unbalance measuring operation on a wheel while it is still fixed to the motor vehicle, the wheel to be measured is generally lifted, for example by means of a jack, to ensure that the wheel can be freely rotated in the unbalance-measuring run. In order to drive the wheel in rotation at the necessary speed, a typical unbalance measuring machine of that kind has a friction wheel which is driven by means of a drive motor and which is pressed against the tire on the wheel to drive it at the necessary speed of rotation for carrying out the unbalance measuring operation. In that situation the drive motor of the machine may be switched on before or after the friction wheel has been brought into contact with the tire.
It is found in some cases that there is no guarantee that the pressure force with which the friction wheel of the unbalance measuring machine is pressed against the tire of the motor vehicle wheel to be balanced is sufficient to transmit the drive torque produced by the drive motor to the wheel, without slippage between the friction wheel and the tire. It will be appreciated however that slippage-free drive for the wheel is desirable in particular to protect the tire from wear which would occur in the event of slippage between the friction wheel and the tire. Slippage of that kind may also occur in the wheel deceleration phase after the drive motor of the machine has been switched off, at the end of an unbalance measuring operation. Furthermore, if the motor is still switched on and the friction wheel is subjected to a braking effect, that can result in the motor being overloaded.